A closed, Rankine cycle power plant is disclosed in U.S. Pat. No. 3,393,515 wherein an organic working fluid is vaporized in a boiler and supplied to a turbine which, together with a generator, is mounted on a common shaft rotatably supported hydrodynamic bearings in a hermetically sealed cannister. Vapor exhausted from the turbine is passed into a condenser which converts the exhaust vapor into condensate at a lower temperature and pressure than in the boiler. Some of the condensate in the condenser is supplied to the bearings and the remainder is returned to the boiler, either directly, if the condenser is elevated sufficiently relative to the boiler or via a pump if the elevation is insufficient. Such a power plant is hereinafter termed a power plant of the type described.
The use of hydrodynamically lubricated bearings in a power plant of the type described insures the presence of a fluid film of lubricant between the metal of the journal and the metal of the bearing. As a consequence, the only friction is that which occurs in the fluid film as it is sheared due to rotation of the journal. By utilizing techniques that insure a supply of liquid working fluid to the bearings prior to start-up of the turbine of a power plant of the type described and by utilizing a hermetically sealed cannister, the bearings will always be lubricated adequately, and their life will be indeterminately long with the result that a highly reliable power plant is established. For reasons concerned with the viscosity of the working fluid at the condenser temperature, the rotational speed of the turbine, and the bearing loading, power plants of the type described have heretofore been limited almost exclusively to relatively low levels of power, typically 1-2 KW. In this power output range, a power plant of the type described is well adapted, and is currently being utilized successfully, for powering unmanned microwave relay stations located in remote regions of the world, wherein the only maintenance over extended periods of time is replenishment of the fuel for the boiler.
The mass flow required to produce such relatively low levels of power can be handled well by a partial admission nozzle system that provides a turbine of relatively small size with a rotational speed that permits the turbine to be coupled directly to the generator. As a result, the turbine wheel and generator can be mounted on a common shaft. While the temperature of the condensate may be as high as 150.degree. F., the viscosity of the working fluid, which may be Freon 12 or a similar organic fluid, is such that the resultant minimum lubricating film thickness formed by the condensate in the bearing is large enough under the conditions of rotational speed and bearing load for conventional metal working techniques to be utilized in fabricating the bearings and journals.
In scaling-up an organic fluid turbine by several orders of magnitude to produce a power plant in the range 150-500 KW, the diameter of the turbine must be increased significantly to provide sufficient cross-sectional area for the nozzles to acheive the required through-put of working fluid. As a consequence, rotor speed must be reduced to maintain rotor stress within allowable limits. For example, a 1-2 KW power plant will operate at around 12,000 RPM while a turbine in the 500 KW range will operate at around one-fourth of this speed. Consequently, a direct coupling between the turbine and rotor can no longer be utilized. Rather, an indirect mechanical coupling, such as a gear train, must be interposed between the turbine and the generator. This requirement creates a number of problems. First of all, in order to utilize the working fluid at the condenser temperature as a lubricant for the bearings of the turbine rotor at the bearing speeds and loadings required, the minimum lubricating film thickness achieved with condensate is so reduced that that sophisticated metal finishing techniques have to be utilized in order to produce the bearings and journals. As a consequence, large scale organic fluid turbines heretofore built require specially finished journals and bearings if condensed working fluid is to be used as a lubricant, or the use of conventional lubricants within the sealed cannister, or the use of ball bearings rather than journal bearings. Neither conventional lubricants nor ball bearings are viable solutions to this problem. Secondly, the viscosity of a typical organic working fluid at the condenser temperature is so low that liquid working fluid cannot adequately lubricate the rubbing surfaces of the mating gears. Therefore, it is conventional to place both the gear box and the generator outside the turbine cannister. This means that the turbine shaft must pass through the cannister reducing the quality of the hermetic seal of the cannister. At best, long term operation will result in the loss of working fluid at the rotary seal through which the turbine output shaft passes, and/or contamination of the working fluid from environmental factors. The advantage of this approach, however, is that conventional lubricants can be used in the gear box; and as a consequence, power plants in the range 150-500 KW have been built and operated successfully by placing the gear box and generator outside the cannister containing the turbine.
The special finishing requirements for the journals and bearings of the turbine increase the cost of manufacture and assembly, and the provision of a rotary seal on the cannister reduces the reliability of the entire system due to loss or contamination of the working fluid. It is therefore an object of the present invention to provide a new and improved lubricating system for an organic power plant which reduces or substantially overcomes the problems described above.